Charakterisierung teilkristalliner und gealterter Polymersysteme mittels Terahertz-Spektroskopie

In den letzten Jahren stieg das Interesse an der zerstörungsfreien Materialanalytik mit Terahertz-Strahlung. In vielen Anwendungsszenarien bietet die Terahertz-Spektroskopie vielfältige Möglichkeiten Materialien zu charakterisieren und zu klassifizieren. In dieser Arbeit wurde das Potential der THz-...

Full description

Saved in:
Bibliographic Details
Main Author: Sommer, Stefan
Contributors: Koch, Martin (Prof. Dr.) (Thesis advisor)
Format: Doctoral Thesis
Published: Philipps-Universität Marburg 2017
Online Access:PDF Full Text
Tags: Add Tag
No Tags, Be the first to tag this record!

During the last decade spectroscopy with THz radiation has gained importance. In different fields of application THz spectroscopy offers a significant potential for material characterization. In this thesis, the ability of THz technologies for the characterization of aged und semi-crystalline polymeric materials is demonstrated. In order to fully evaluate the possibilities the technology offers to characterize different material properties, a variety of selected thermoplastics were investigated with THz spectroscopy. The spectra of the absorption coefficient and the refractive indices in the bandwidth of 0.2 up to 4.0 THz confirm the overall high transparency of the different polymers. Some distinct characteristic absorption features are highlighted out and the molecular origin is discussed. In greater detail the spectra of the aromatic polyesters PET and PBT are discussed. The spectroscopic measurement in the THz frequency range shows two different absorption lines for each polymer. Furthermore the dependency on crystallinity and the crystal structure is discussed: the first peak, observed at 2.5 THz, is believed to represent a phonon-like vibration of the phenyl groups as the absorption peak at 3.1 is believed to originate from a coupled torsional vibration of the carbonyl groups. Angle resolved polarization sensitive THz measurements of differently arranged crystal forms of PBT support this assumption. Additionally these measurements show the possibility to distinguish between different kinds of polymer crystals. In the same way as x-ray techniques, like WAXS and SAXS, also the THz measurement allows to extract crystallographic information to identify the crystal structure. To achieve a crystal form of polyester in most cases an isothermal process is induced. The so called isothermal crystallization was monitored with THz spectroscopy. In-situ and static measurements on well-defined samples were performed and the extracted optical parameters were compared with the degree of crystallinity, extracted from destructive thermal DSC-analysis. In this thesis a direct correlation between the optical parameters (e. g. absorption coefficient) and the kinetics of the isothermal crystallization has been established by using the theory of Avrami. The change in the absorption properties in the THz frequency range of PET during the crystallization has been used to emulate the kinetics. Particularly the degree of crystallization is an important parameter in industry, because the mechanical properties of the polymer are linked to the crystallinity. The THz-measurements of different PE-HD samples show the potential of a correlation between the degree of crystallization and the optical parameters. A linear correlation between both parameters could be determined. Another important parameter, which influences the mechanical properties, is the molecular orientation. Polarization sensitive measurements in transmission and reflection geometries show that the orientation of molecules can be calculated by using a simple mathematical model based on the refractive index. Finally the potential of THz spectroscopy towards the detection of aging effects of polymers was investigated. Different thermosets were thermo-oxidative aged at different temperatures and the optical parameters at THz frequencies were determined. Additional measurements with the FTIR spectroscopy or mechanical tests were carried out to characterize the samples with established methods. The change in the optical parameters of PVC-U, POM, PA 6.6 and PE-HD could directly be correlated with chemical effect of aging. An important aging effect is the evaporation of plasticizer in PVC-P. During thermal treatment the plasticizer, which can be hazardous for humans, evaporates and the material gets frangible. In the case of safety critical aspects it is relevant to detect the evaporation process and to determine the residue concentration of plasticizer. In this thesis a direct correlation between the plasticizer concentration of DINP or ELATUR with the optical parameters of the absorption coefficient and the refractive index could be determined. An additional principle component analysis applied to the spectral data showed the possibility to distinguish between both types of plasticizer. All studies performed in this thesis can show the potential of THz spectroscopy. For the development of industrial proofed and standardized methods the THz systems need further development within the next years. Especially the robustness and the fault liability have to be improved. Although the THz spectroscopy will never be the universal method for material characterization, the results of this thesis show that the THz spectroscopy can be a very useful additional non-destructive, contact-free tool for obtaining material parameters.